There are many devices used underwater that require accurate synchronized clocks. One example, described in U.S. Pat. No. 8,009,516, entitled “Underwater Acoustic Positioning System and Method and Underwater,” the teachings of which are incorporated herein in their entirety, requires all devices to have accurate synchronized time in order for an autonomous underwater vehicle to be able to precisely determine its position underwater. A further example includes underwater seismic arrays that have thousands of recorders spread on the seafloor logging the echo data from common sound waves bouncing off rock layers under the seabed. These systems are typically synchronized to the GPS Atomic clock using the PPS output of GPS receivers in air. Underwater radio waves at these frequencies can't propagate any useful distance.
There are very small oscillators with varying degrees of accuracy that have lower and lower levels of drift correlated with increasing cost. These form the basis for timing in clocks by counting the cycles of oscillation. Table 1 below provides examples of conventional parts illustrating the clock accuracy in parts per billion and the resulting error in meters of sound travel. One design goal could be to have no more than 0.2M of sound travel error induced by the timing. The $100 oscillator would be within this specification for 4 hours while the $2000 atomic clock oscillator will be in specification for 941 hours.
TABLE 1FrequencySoundAccuracy inApprox costClock DriftTravelOscillatorppbin USDin 24 hoursDistanceASV-100000$1  8640 ms12,900 M10.000MHZ-E-TDV75D-010.0M1000$10 86.4 ms  129 MOH300-10$100 0.864 ms  1.2 M61003CV-010.0MSA.45 Chip0.04$20000.0034 ms0.0051 MScale AtomicClock
If a simple and effective method were available to synchronize these clocks every four hours, the $100 oscillator could operate within specification for as long as desired. This can be compared to operation of the $2000 atomic clock without such a method, which will drift out of specification after 941 hours. The method would thus allow the less expensive oscillator to provide the required accuracy—a significant advantage where many clocked devices are required.
In another way to look at the problem, deployment of the $2000 clocks with synchronization every 4 hours can limit the drift to 56 micro seconds for as long as desired. A practical example would be a system based on U.S. Pat. No. 8,009,516 where station keeping beacons are mounted permanently across a harbor as a full time navigation resource, they can be battery powered and can use the SA.45 atomic clocks. Synchronization once every 2 weeks would limit the time error in distance to about 7 cm. This could provide a practical way to place a multitude of standalone battery powered devices for use as permanent underwater GPS like facility.